This invention relates to the field of cyclins.
The cyclins are a class of eukaryotic proteins that were originally identified by their cyclic accumulation and destruction at defined points in embryonic cell cycles (Evans et al., Cell 33:389-396, 1983). They bind to and are essential for activation of cdc2 protein kinase (reviewed in Murray et al., Science 246:614-621, 1989; Nurse, Nature 344:503-508, 1990; Draetta et al., Cell 56:829-838, 1989). At present, the cyclins can be divided into three families on the basis of their kinetics of oscillation across the cell cycle, their amino acid sequences, and, in some cases, genetic experiments in yeast that determine when their functions are needed (reviewed in Nurse, 1990; Nasmyth, Cell 63:1117-1120, 1990; Westendorf, J. Cell Biol. 108:1431-1444, 1989). The B-type "mitotic" cyclins drive cells into mitosis; their sequences are conserved from yeast to human (Nurse, 1990; Westencorf et al., 1989; and Pines et al., Cell 58:833-846, 1989). The A-type cyclins, which are less well understood, may act earlier in the cell cycle (Minshull et al., EMBO J. 9:2865-2875, 1990; Pines et al., Nature 346:760-763, 1990; Swenson et al., Cell 47:861-870, 1986). The recently described CLNs (or "G1 cyclins") of budding yeast are thought to perform analogous functions by interacting with cdc2 homologues at START, driving cells into S-phase (Nasmyth, 1990). A, B, and CLN cyclins may act as stage-specific regulators of progress across the cell cycle by conferring selective substrate specificity upon cdc2 kinase (Minshull et al., 1990) or by selectively targeting cdc2 to different intracellular compartments.